Polycarbonate resins, due to their many advantageous properties, are widely used in industry and commerce. One of their uses is as transparent glazing materials for windows, windshields, and the like. While polycarbonate resins are easily fabricated into the desired shape and have excellent physical and chemical properties, such as being less dense and having greater breakage resistance than glass, they have a relatively low abrasion and chemical solvent resistance, and like many other organic polymeric materials are subject to degradation by ultraviolet radiation.
In order to overcome this relatively low resistance to abrasion and chemical solvents various protective coatings which possess greater abrasion and chemical solvent resistance than polycarbonate resins have been applied onto the surface of polycarbonate articles. However, in order to qualify as a successful coating material for polycarbonate resins there are several requirements that the prospective coating material must meet. The coating material must be harder and more solvent resistant than the polycarbonate resin. The coating material must be compatible with the polycarbonate and must not degrade the polycarbonate such as by crazing the polycarbonate or otherwise adversely affecting the properties of the polycarbonate resin. The coating material must durably adhere to the surface of the polycarbonate. U.S. Pat. Nos. 3,451,838; 3,986,997 and 4,027,073 disclose organopolysiloxane coating compositions and techniques for the application of these organopolysiloxane coatings onto polycarbonate surfaces. While these organopolysiloxane coatings have many desirable properties, e.g., they are hard, abrasion and solvent resistant, and are compatible with the underlying polycarbonate, these organopolysiloxane do not in all instances possess the requisite degree of adhesion to and durability on the polycarbonate. In order to improve the adhesion of these organopolysiloxane coatings to the polycarbonate substrate it has been suggested to use adhesion promoting primer layers between the organopolysiloxane and the polycarbonate. However, the use of a primer layer adds an additional degree of uncertainty and complexity to this already difficult and largely imperical area of coating technology. In order to function effectively the primer layer must not only increase the adhesion of the organopolysiloxane coating to the polycarbonate but must also be compatible with both the polycarbonate and the organopolysiloxane. U.S. Pat. No. 3,707,397 describes a process for providing a hard coating on, inter alia, polycarbonate resin by priming the polycarbonate surface with an adhesion promoting thermosettable acrylic polymer and applying onto this primer a thermosettable organopolysiloxane. An article produced by this process, while processing acceptable initial adhesion of the organopolysiloxane to the polycarbonate, suffers from the disadvantage that upon prolonged exposure to weathering, particularly to sunlight, the organopolysiloxane coating generally tends to lose its initial good adhesion to the polycarbonate resin substrate. Furthermore, the abrasion resistance of the coated article is generally dependent upon the thickness of the thermoset acrylic polymer primer layer. The abrasion resistance of the coated article generally decreases as the thickness of the primer layer increases. The deterioration of the adhesion of the organopolysiloxane coating to the polycarbonate substrate upon exposure to weathering is rectified to a certain degree in articles produced according to the methods disclosed in U.S. Pat. Nos. 4,197,335 and 4,207,357. In the processes disclosed in these two patents the polycarbonate substrate is primed with a primer composition comprising an emulsion of a thermosettable acrylic polymer, water and a hydroxy ether, and the organopolysiloxane containing top coating is then applied onto the primed polycarbonate substrate. However, the abrasion resistance of the coated articles thus produced is still generally dependent on the thickness of the primer layer.
While these prior art methods generally provide a protective coating for the polycarbonate article effective to protect it from abrasion and chemical solvents, they do not provide protection against degradation by ultraviolet radiation. It would appear at first glance in view of the prior art that there are three methods of protecting the coated polycarbonate article from degradation by ultraviolet radiation: (1) incorporating an ultraviolet radiation absorber into the silicone topcoat; (2) Incorporating an ultraviolet radiation absorber into the thermoset acrlic polymer containing primer layer; and (3) incorporating an ultra violet radiation absorber into the polycarbonate resin itself. However, upon closer scrutiny and in light of the mostly empirical knowledge gained in this area each of these three methods turns out to contain certain inherent problems. Incorporating an ultraviolet radiation absorbing compound into the silicone top coat generally results in a decrease in the abrasion resistance provided by the silicone. The greater the amount of ultraviolet radiation absorbing compound present in the silicone top coat, the greater the loss of abrasion resistance provided by the top coat. Thus, if the silicone top coat contains sufficient amounts of ultraviolet radiation absorber to effectively protect the underlying polycarbonate resin from degradation by ultraviolet radiation its abrasion resistance is generally unacceptably lowered. If the ultraviolet radiation absorbing compound is incorporated into the thermoset acrylic polymer containing primer layer the aforediscussed relationship between primer thickness and abrasion resistance of the silicone top coat comes into effect. In order to effectively protect the polycarbonate resin from ultraviolet radiation the primer layer must contain relatively large amounts of ultraviolet radiation absorbing compounds. But in order to contain these relatively large amounts of ultraviolet radiation absorbing compounds the thickness of the primer layer must be increased. However, this increase in thickness of the primer layer which is required to accomodate the necessary amounts of ultraviolet radiation absorbers results in a corresponding decrease in the abrasion resistance of the silicone top coat. Thus in both of these methods protection against ultraviolet radiation is provided only at the expense of protection against abrasion.
The third method of providing protection against ultraviolet radiation involves incorporating the ultraviolet radiation absorbing compound directly into the polycarbonate resin. This method involves either (i) blending the absorber with the bulk polymer, or (ii) impregnating the surface layers of the resin with the absorber. Blending the absorber with the bulk polymer results in the absorber being distributed throughout the entire polymer system. This procedure is both uneconomical, as these ultraviolet radiation absorbing compounds are usually quite expensive, and not completely successful. Since most of the absorber resides in the polymer's interior instead of at the surface where it is most needed, much of the harmful ultraviolet radiation penetrates and deteriorates the surface layers of the polymer structure before reaching the majority of the interiorly distributed absorber. Furthermore, since the concentration of the absorber in the resin is limited by the degree of compatibility of the absorber with the resin, using sufficiently high concentrations of absorber effective to provide adequate surface protection generally tends to adversely affect the physical properties of the polymer. In the surface impregnation technique the ultraviolet radiation absorber resides in the surface regions of the polymer where it is most needed. Examples of typical surface impregnation techniques generally include applying the ultraviolet radiation absorber from a stabilizing solution containing a compound which is aggressive towards the polycarbonate and tends to swell or soften the resin thus enabling the absorber to diffuse into the swelled and softened surface of the polycarbonate layer, as disclosed in U.S. Pat. Nos. 3,892,889 and 4,146,658; melting the ultraviolet radiation absorber on the surface of the polycarbonate resin and allowing the molten absorber to diffuse into the surface layers of the resin, as disclosed in U.S. Pat. No. 3,043,709; and immersing the polycarbonate resin in a stabilizing solution containing an ultraviolet radiation absorbing compound wherein the compound is more soluble in the polycarbonate resin than in the stabilizing solution, as disclosed in U.S. Pat. Nos. 3,309,220 and 3,594,264.
However, the very feature which makes the surface impregnation method appear attractive, i.e., that the ultraviolet radiation absorber is distributed in the surface layers of the polycarbonate resin where it is most needed, also makes this method appear to be untenable to one skilled in the art when this method is used in conjunction with the application of a protective coating onto the polycarbonate surface. The complexity and problems associated with providing a protective coating which adheres tenaciously and durably to a polycarbonate surface have been discussed above. The modification of a polycarbonate surface by incorporating therein an ultraviolet radiation absorber, as is done by the surface impregnation technique, adds yet further complications to this already complex area of adhering protective coatings to polycarbonate. It is well known to those skilled in the coating art that modifying the surface of polycarbonate by incorporating an additive therein has generally unpredictable and often adverse effects upon the physical properties of the polycarbonate surface. These effects upon the polycarbonate surface depend upon the particular additive employed. It is generally quite well known that the incorporation of certain additives into the surface areas of polycarbonate resin often results in the deterioration of both initial adhesion and durability of adhesion between the polycarbonate surface and a protective coating applied onto this surface. In view of this one skilled in the art would generally be led to conclude that the incorporation in the surface layers of a polycarbonate resin of an amount of ultraviolet radiation absorbing compound effective to protect the polycarbonate from degradation by ultraviolet radiation would deleteriously affect the adhesion of a protective coating to this modified polycarbonate surface.
There thus exists a need for a means of simply and effectively protecting a polycarbonate resin from degradation by ultraviolet radiation, from surface abrasion, and from attack by chemical solvents. The present invention provides such a method as well as the article produced by this method.